Vanguard core funds are a very popular set of mutual funds. Two of them, VFINX and VFIAX, are extremely similar. They both hold the 500 largest stocks in the US equity market. They both have the same size and content, but VFIAX has an expense ratio of 0.04% and VFINX has an expense ratio of 0.14%. In previous years this made sense - VFIAX required a minimum investment of $10,000 and VFINX only needed $3,000. As with many financial operations, a larger sum of money enabled a cheaper expense rate. However, recently VFINX closed to new investors, and VFIAX dropped its minimum to $3,000 (or $0, according to my own bank).
As an amateur investor who isn't familiar with pricing structures and mutual funds, I can think of no reason why Vanguard would maintain two identical funds, one being more expensive than the other for the exact same content. Shouldn't any reasonable investor drop all of their holdings in VFINX and move to VFIAX immediately? Even if you only had $3000 and were forced to do VFINX, now you can invest in VFIAX at a lower cost. What am I missing?